Selective laser melting is a method allowing the manufacturing of a molded body, for example a prototype of a product or of a component, according to three dimensional CAD data of a model of a molded part by deposition of layers of a material as a powder. Several layers of the powder are successively deposited on each other, so that each powder layer is heated to a temperature determined by a focused laser beam of high power (from 200 W to a few kW) applied on a given area of the powder layer corresponding to a determined cross-section area of the model of the molded body, before depositing the next layer. The laser beam is guided above each powder layer in accordance with the CAD data of the determined transverse surface of the model, so that each powder layer is attached to the underlying layer. By repeating the provision of powder and of its melting by the laser gives the possibility of gradually thickening the part and obtaining the desired shapes.
An example of such a laser selective melting method has notably been described in documents FR 2 970 887 and U.S. Pat. No. 6,215,093.
In order to obtain a part with metallurgical quality and a dimension observing the tolerances notably imposed in the field of turbine engines, it is necessary that the energy density of the laser beam remains constant in the manufacturing plane (the powder bed) and during the period of exposure to the laser.
The energy density depends on three parameters: the power, the velocity and the size of the laser beam.
Presently, the monitoring of the energy density of the laser beam is obtained indirectly by independently measuring these three parameters. Nevertheless, this monitoring has the drawback of requiring individual measurements which are successively carried out by different apparatuses. In addition to being long and difficult to apply in industry, this monitoring of the energy density of the laser beam is not robust, the measurements undergoing the risk of being distorted by drifts of the apparatuses used. These apparatuses are further specific and often require training and particular skills in order to be used, are further expensive, fragile and long to apply and have to be periodically checked. Finally, the whole of the manufacturing space is not accessible for allowing these measurements.
In document EP 1 466 718, a method for controlling the temperature on a target area (such as a powder bed) has been proposed, from an image of the target area acquired by means of a thermal vision system, such as an infrared camera. The thereby determined temperature is then compared with a desired temperature, which gives the possibility of improving the control of the overall temperature of the powder bed. However, this method by no means allows determination of whether the energy density of the laser beam is stable, or whether one of the parameters on which it depends is unstable. Indeed, the question is only of adapting the temperature of the laser beam so as to avoid temperature differences at the powder bed which would be detrimental to the quality of the part which one seeks to manufacture.
Document DE 103 20 085, as for it, describes a method for manufacturing a part by selective melting, during which the light density is adapted so as to improve the final density of the part. Adaptation of the light density is in particular adapted by means of a CCD camera, which measures the thickness of the powder to be melted, and a pyrometer, which determines its temperature. This document therefore does not deal with the control of the stability of the energy density of a laser beam.
Finally, document DE 10 2010 027910 deals with a method for manufacturing a part by laser melting, during which the power of the laser is regularly measured in order to detect possible deviations with respect to expected power values. This document therefore only proposes measurement of one of the parameters playing a role on the energy density of the laser bream, the other parameters not being determined.
None of these documents therefore proposes a reliable and fast means for detecting a drift of the parameters of the energy density in a simple way and for moderate cost.